Best-effort type speech communication represented by VoIP (Voice over IP) is commonly used in recent years. Transmission bands are generally not guaranteed in such speech communication, and therefore some frames may be lost during transmission, speech decoding apparatuses may not be able to receive part of coded data, and such data may remain missing. When, for example, traffic in a communication path is saturated due to congestion or the like, some frames may be discarded, and coded data may be lost during transmission. Even when such a frame loss occurs, the speech decoding apparatus must compensate for (conceal) the lacking voice part produced by the frame loss with speech that brings less annoying perceptually.
There is such a conventional technique for frame loss concealment that applies different loss concealment processing to voiced frames and unvoiced frames (e.g., see Patent Document 1). When a lost frame is a voiced frame, this conventional technique performs such frame loss concealment processing that repeatedly uses parameters of the frame immediately preceding the lost frame. On the other hand, when the lost frame is an unvoiced frame, the conventional technique performs such frame loss concealment processing that adds a noise signal to an excitation signal from a noise codebook, or randomly selects an excitation signal from the noise codebook, thereby preventing generation of decoded speech that brings perceptually strong annoying effects which are caused by consecutive use of an excitation signal having the same waveform.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI10-91194